TfR^-/- chimeras display a differential ability to produce T and B cells in secondary lymphoid organs on a RAG2^-/- background. Nucleated cells from 7-week-old chimeras and control animals were stained with Ly9.1, B220, and Thy1.2 and analyzed by flow cytometry. Cells (200 000-500 000) were counted for these analyses. Percentages of cells in each quadrant are indicated. (A) Splenic cells. (B) Lymph node (axial, brachial, inguinal, and mesenteric) cells. All TfR^-/- chimeras show few, if any, Ly9.1⁺Thy1.2⁺ cells. Only those TfR^-/- chimeras with TfR^-/- cells in the bone marrow (no. 30 and no. 32) display a small number of Ly9.1⁺B220⁺ cells. The TfR^-/- BM-negative chimera shown is representative of the other 4 TfR^-/- BM-negative chimeras obtained.